Digital telecommunication networks, more particularly, pulse code modulation (PCM) time division multiplex (TDM) telecommunication networks having clock pulse oscillators provided in the network nodes thereof, e.g., in switching centers, have the function of compensating differences in bit frequencies between signals transmitted on various digital circuits leading to a network node. Such compensation is a prerequisite for correct switching of telecommunication circuits in the telecommunication central offices of such a digital network. Various principles underlying a solution are known for such a compensation of differences between bit frequencies (Proc. IEE, 113 (1966) 9, 1420 . . . 1428, 1421; "Reports on Telephone Engineering", Siemens A. G., 5 (1969) 1, 48 . . . 59).
In asynchronous operation (heterochronous operation) each PCM/TDM central office of a PCM/TDM telecommunication network has a special independent clock generator. Each central office receives time division line ends in a pulse-frame storage, the storage capacity of which corresponds to the number of bits per pulse frame and in which the binary words received are retained until they fit into the pulse frame of the PCM/TDM central office involved (at the same time the pulse-frame storage carries out a frame compensation).
In quasi-synchronous operation (using dummy bits), the PCM/TDM central offices of a PCM/TDM network have their own independent clock generators, but the information bit frequency, i.e., the mean number of information bearing bits per second is made equal for all PCM time-division multiplex central offices of the whole network. This is accomplished by compensating for the difference between the bit clock frequencies of the individual PCM/TDM central offices and the uniform information bit frequency through the insertion of dummy bits.
In servosynchronous operation (homochronous operation or master-slave operation), a central clock generator determines the bit frequency of the individual PCM/TDM central offices of a PCM/TDM telecommunication network.
Finally, in autosynchronous operation the individual PCM/TDM central offices have individual clock generators which, however, are not independent of one another, but lock on to one another, for example, in accordance with the phase averaging principle.
For the aforementioned form of operation, it is old and well known in the art to allocate in the individual central offices of a PCM/TDM telecommunication network, to the TDM lines incoming therein, phase discriminators which receive and compare a pulse train corresponding to the respective incoming line clock pulse frequency, as well as a pulse train corresponding to the office clock frequency of the respective central office. The output signals from the phase discriminators correspond to the respective phase shift between the respective line clock pulses and the office clock pulses; the output signals are combined in a sum or average-determining element to produce a control signal for controlling the frequency of the respective central office clock oscillator. Phase shifts may be caused by different clock frequencies of the clock oscillators provided in the individual central offices of the telecommunication network and/or by variations in them.
It is a known practice (cf. Electronics and Communications in Japan, 49 (1966) 11, 165) to employ a pulse train corresponding to the respective line clock bits or the central office clock bits and having a pulse repetition rate representing a submultiple of the clock bit frequency. This may be effected such that in the individual central offices of a PCM/TDM telecommunication network from the incoming PCM/TDM lines the line clock bits of the individual incoming PCM/TDM lines are extracted by means of flywheel circuits from the received PCM signals, whose phase shifts relative to the office clock bits of the central office involved cause the control of the clock oscillator supplying the central office clock bits. Incoming line clock bits and central office clock bits are routed to two frequency dividers starting the frequency division and are spaced preferably at 180.degree. relative to each other (regeneration of the reference phase). A phase comparison between the frequency divider output pulse trains is carried out by means of a bistable circuit. The DC average of the output signal of the bistable circuit is proportional to the phase difference and, thus, proportional to the integral of a frequency difference, viz. the difference between line clock frequency and the central office frequency.
The output signals of all bistable circuits allocated to individual lines are added by means of resistances for averaging and smoothed by a RC network. The capacitor voltage from the RC network regulates the clock frequency of the central office oscillator by varying the capacitance of a capacitance diode.
The reset pulses from the central office oscillator frequency divider act on the counting inputs of the individual bistable circuits allocated to the two bistable circuit fields; if a line clock bit signal breaks down, the associated bistable circuit runs as a counter having a duty cycle ratio of 1:1. This leads to a control voltage equal to an agreement between line clock frequency and central office clock frequency.
Two or more of the above principles may also be applied simultaneously. Thus, it is well known (U.S. Pat. No. 3,684,837) that in the individual network levels of a multi-level telecommunication network or in the individual networks of a telecommunication network comprising two or more networks there is mutual synchronization of the clock oscillators in the autosynchronous mode, while a controlled synchronization in the servosynchronous mode is provided between the network levels or between the individual communication networks over two or more lines.
With directed synchronization of a digital telecommunication network, through an additional network, it is desirable that such synchronization be effected without restrictions, as far as the network configuration is concerned, particularly with a view to increasing the number of communication lines linking the communication networks together or, where applicable, merely serving the clock pulse transmission. However, insertion of additional communication lines between a given digital telecommunication network and other such networks does not lead, without special provision, to an improvement of the synchronizing effect on the network to be synchronized from the additional network. Instead, it may lead to an undesirable momentary frequency variation within the communication network.
The aforementioned variation may be prevented (U.S. Pat. No. 3,684,387) by causing the frequency division effected for obtaining a pulse train corresponding to the line clock bit of the inserted communication line in accordance with a frequency division factor to start with the above mentioned spacing at 180.degree. relative to the frequency divider of the central office clock oscillator in question. However, a synchronizing influence on the digital communication network to be synchronized is exerted, at least for the time being, as before, only over the existing line or lines between the synchronizing additional communication network and the digital communication network to be synchronized due to the phase difference or differences prevailing therein between the pulse train corresponding to the respective line clock bit rate and the pulse train corresponding to the respective line clock bit rate and the pulse train corresponding to the respective office clock bit rate. The newly inserted communication line remains without effect for the time being, due to the 180.degree. phase difference. Subsequently, the network being synchronized can be so synchronized using a smaller control range than would otherwise be necessary, since it is not very likely that the phase discriminators of all communication lines extending from the synchronizing additional communication network to the digital communication network to be synchronized operate in at least substantially identical operating points. It can, therefore, concurrently be driven to full output. However, such a small frequency control range may be inadequate if there is a large no-load line frequency difference between the non-externally synchronized digital communication network and the additional communication network.
An object of the invention is, therefore, to provide a mean and method for removing the restrictions enumerated above and to provide for the efficient synchronization of a digital communication network.
Another object of the invention is to provide means and method for the aforementioned synchronization using one or more additional communication line or lines, by means of an extra communication network connected to said digital communication network over one or more communication lines.